Hi there Matthias,
I'll keep it short, no filler and fluff. I have 14+ years of programming and after reading your requirements, I'm 101% positive I can help you with your task.
I'm so confident because I would not risk a new profile with something I can partially do. In fact, just to test my programming knowledge, you can throw any kind of problem at me right now (not necessarily related to your project) and I'll solve it for you.
For this project, we'll create a post type where you will write the questions and the possible answer for each question and another post type for the tests so you can create multiple tests. To each question you'll assign a test and the script will randomly choose from the questions you gave a max number of questions for that specific test. I already created a similar form but it was much more complex with multiple types of questions (select, radio, single field box for answer etc).
When you're available, please ping me with more details so we can get this done ASAP.
Looking forward to helping your reach your goals and build a great business relationship.
Regards,
Kalin